Nuclear magnetic resonance (NMR) is a phenomenon which occurs when the nuclei of certain atoms are subjected to a magnetic field in the presence of a second, perpendicular, oscillating magnetic field, and this phenomenon is commonly employed in NMR imaging and spectroscopy to study the physical, chemical or biological properties of samples of matter. This may best be understood by reference to FIG. 1.
Many species of nuclei contain non zero spin and an associated magnetic dipole moment, and it is this which gives rise to nuclear paramagnetism and hence nuclear magnetic resonance (NMR). As shown in FIG. 1, when a static magnetic field H0 is applied to a sample containing nuclear spins, the spins of the various nuclei become aligned in the direction of the Z axis. As a result of the underlying quantum mechanics of the system, the spins will precess around the Z axis at some angle θ, with a rate ω0 given by the Larmor precession equation:ω0=γH0 where γ is the gyromagnetic ratio for the sample under consideration. If an additional radio frequency (RF) field H1 is then applied in a direction perpendicular to the static magnetic field H0 at a frequencyωRF=ω0 the angle θ will be increased due to resonant absorption.
There are two conventional forms of NMR apparatus, known respectively as continuous wave NMR apparatus and pulse NMR apparatus, the signal detection in each case being by means of an inductive sensor as shown in FIG. 2. This inductive sensor comprises a resonant circuit 10 used to probe a sample 12 that is situated in a static magnetic field H0. In its simplest implementation, as illustrated, this circuit is used both to supply, and to read the output generated by, an excitation signal applied to the sample 12. For this purpose, RF power is supplied by an RF source 14, which is inductively coupled by means of a coil 16 to the sample 12.
Continuous Wave NMR
In the known continuous wave NMR apparatus, the static magnetic field H0 is swept slowly in amplitude, and whenω0=ωRF absorption occurs and a dip in the signal voltage across the inductive coupling is observed. The same result may also be achieved by fixing the amplitude of the static magnetic field and sweeping the RF frequency. The signal voltage requires a large amount of amplification in order for the dip to be manifest, and is therefore applied to an amplifier 18 for output.
In practice, the slow variation of the static magnetic field H0 is generally achieved by adding a time varying field to the static one. This is often accomplished using additional coils.
FIG. 3 shows a conventional magnetic continuous wave NMR absorption dip acquired using the apparatus of FIG. 2. The triangular ramp A represents a time varying signal (modulation) added to the static magnetic field H0, and the lower trace B shows the absorption dip for a sample of liquid glycerine. As expected, we see two dips per period of the modulation waveform, one for each time that the static field generates the result ω0=ωRF.
Pulse NMR
In pulse NMR apparatus, the RF field H1 consists of a short pulse of RF power, the RF pulse being applied to the sample 12 with an amplitude chosen such that the angle θ of the precessing spins becomes 90°, thereby tipping them from the Z plane into the X-Y plane. This is shown in FIG. 4. Clearly this corresponds to the maximum amplitude of the precession, and a pulse producing this deflection is called a 90° or π/2 pulse. A 180° or π pulse would simply flip the spins to the Z axis antiparallel to the applied static magnetic field. The pulse length is designated as:                π/2 when θ is rotated by 90°        π when θ is rotated by 180°        
Once the RF pulse has ended, the spins will continue to precess but the amplitude of the precession, the angle θ, will decrease with time due to interaction of the spins of the nuclei with one another. This gives rise to what is known as the free induction decay (FID) signal from the precession of the spins, an exponentially decaying sinewave generally at the Larmor precession frequency. It is the FID signal which is detected in pulse NMR.
In a variation of this arrangement, the applied RF field H1 is selected to have a frequency close to but not quite satisfying the Larmor precession equation ωRF=ω0. Mixing then occurs, which gives rise to a lower frequency output signal, which is easier to amplify and filter. This process is known as nutation.
Pulse NMR apparatus employs a sensor circuit generally the same as that shown on FIG. 2, with the exception that in this instance the RF source 14 supplies an oscillating pulse of short duration while the static magnetic field H0 is of constant amplitude. FIG. 5 shows a typical NMR π/2 RF pulse and the resulting FID signal occurring following the pulse.
The known inductive arrangements have been in use for many years. However, they suffer from a number of disadvantages.
In particular, in both the continuous wave NMR apparatus and the pulse NMR apparatus, the radio frequency excitation signal is applied to the sample inductively by means of a coil, and the nuclear precession signal is read out via inductive coupling. This necessarily introduces constraints, both physical and electronic on the performance of the system. Physically, the coil geometry may be restricted or determined by the nature and size of the sample. Further, an inherent electronic problem associated with the traditional approach is that the transmitter coil couples to the receiving coil inductively, leading to saturation of the receiver amplifier and a subsequent dead time during which the system recovers from overload. Numerous measures have been taken to alleviate this problem, including ensuring that the transmitter and receiver coils are perpendicular to each other, adding diode protection circuits, and the use of quarter wave transmission lines. However, despite these measures the inherent difficulty remains that a large amplitude RF transmitter pulse must be coupled to the sample, and that a small but significant proportion of this will couple inductively to the receiver coil and hence saturate the amplifier system.